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Underground

Convergence Based Roof Support Design

Underground » Strata Control and Windblasts

Published: May 17Project Number: C24015

Get ReportAuthor: Terry Medhurst | PDR Engineers

The aim of this project was to develop a roof support design approach that takes account of differing roof conditions, effect of support type and stiffness that can be used for mine design. An analytical framework is presented that provides a measure of both support load and roof convergence which can be matched and updated against roof monitoring data. It is based on beam-column principles and incorporates bending, immediate roof failure and shear. The model relies upon inputs from the Geophysical Strata rating (GSR), roof bolt characteristics including pull-out stiffness/load, in-situ stress ratio and unconfined compressive strength (UCS).

 

A series of investigations were completed at various mines to develop and calibrate the roof beam model. These investigations were completed as part of the project and/or with additional support provided directly from the mine. In addition to the work undertaken in C22008, several sites were investigated that included instrumented roadways. A summary of test sites is outlined below:

· Assessment of trial for introduction of Goliath cables at Grasstree Mine to examine effects of different cable spacing and capacity in good strata;

· Strata management review and TARPs input at Oaky North Mine including strata characterisation, support density, roof convergence inter-relationship;

· Roof stability assessment at Grosvenor Mine to investigate influence of weak strata and thin coal roof at depth;

· Assessment of varying roof conditions and influence of laminated strata on roof stability at Appin West Mine;

· Longwall install road analysis at Oaky No. 1 Mine with changing roof quality in split zone with large span; and

· Depth/stress roof quality variation assessment at Blakefield South subjected to multi-seam loading conditions and varying topography.

 

By examining comparative datasets it became apparent that the ability to incorporate the GSR/σH ratio would be a useful input to quantify the effect of varying depth/stresses using the roof beam model. A procedure has been developed to calibrate beam end constraints and section properties relative to the stress and strength conditions present.

 

A common precursor to stress related damage in roadways is slip on bedding surfaces either on one or both sides of the roadway depending upon its orientation to the principal stress direction. The model included the ability to assess the influence of bedding plane shear on roof stability and the subsequent effect on roof convergence. Using this model, estimates of roof convergence for various heights of softening (or surcharge loading) above a roadway can be obtained for a given support pressure and plotted on a curve, i.e. ground response curves.

 

The use of ground response curves can be difficult to interpret since appropriate heights of softening, convergence and/or triggers need to be chosen from the intersection points on the curves. A mathematical solution was therefore developed to find relevant intersection points from the roof beam analysis. These intersection points can then be used to develop a set of design stability curves to assess the potential for shear failure (Factor of Safety) and allowable roof deformation (Serviceability Factor) due to bending or roof sag in the roadway.

 

A unique set of stability curves can be derived for any support configuration, stress conditions and strata characteristics. This provides the ability to assess roof stability according to each particular location. Several investigative examples are provided to demonstrate both the development and capabilities of the roof beam model. These include methods to review/update TARP triggers, a means to alter support types and spacing, staged excavation for wide spans along with changes in roof characteristics and stress conditions. Special cases for coal roof and highly laminated strata are also considered.

 

In the case of coal roof, corrections are applied to the method in consideration of the differing stress regime and strength characteristics that are unique to coal seams and their effect on the formation of a roof beam. Similarly, an approach was taken in which a correction would be applied to the GSR for the presence of laminated clays assuming the degree of laminations would vary according to the porosity and corresponding clay content at that site. This method proved successful for use in the roof beam model, but further work is required. It would appear that a specific porosity-clay relationship for each site (geological domain) would yield a suitable correction factor.

 

In summary, the method is aimed at providing a practical tool for support design and assessment, and has been tested at a number of Australian underground operations. It is intended that it can be used as part of the suite of available design tools for support design. Distinct mechanisms such as wedge failure, or localised stress influences around geological structure are not applicable to this method and need to be treated separately for design and within the strata management framework. Estimates of Geophysical Strata Rating (GSR) are inherent to the formulation which include estimation of beam section properties, roof stiffness, influence of stress on beam fixity and installed support stiffness. An appropriate estimate of GSR is therefore critical to the reliability of the method. It is also intended to extend the approach to gateroad stability under longwall abutment loading conditions.

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